Description: The major function of the animal core is to provide vitamin a depleted (VAD) synchronized testes for Projects 2-4. The core will be responsible for vitamin depletion, retinol replacement, and staging of rats. The core will perform assessment and verification of synchronized testes, and maintain records of rat experiments. The rats will be housed and cared for by the Hopkins animal facility and a significant portion of the budget reflects the per diem care costs for the rats. The animal core directory estimates that only 70% of the VAD rats will become synchronized. Project 1 will utilize 70 rats, and projects 2 and 3 will share animals and utilize a total of 24 rats per year. The number of rats required for isolation of Sertoli cells is estimated to be 148 rats per year. Considering a 30 percent failure rate in VAD synchronization, the estimated number of rats required per year will be 352.